Typical examples of an infrared signal processing circuit are: remote controllers of home electric appliances and peripheral devices of personal computers, each of which performs data communication in compliance with IrDA (Infrared Data Association) standard or IrDA Control standard.
For example, a conventional infrared remote control receiver 110 includes a photodiode chip 101 and a reception chip 108 as shown in FIG. 23. The photodiode chip 101 converts a remote control transmission signal received from an infrared remote control transmitter (not shown) into a current signal Iin. The reception chip 108 includes: a current-to-voltage-conversion circuit 102 for converting the current signal Iin having been generated into a voltage signal; an amplifying circuit 103 for amplifying the voltage signal having been generated; a bandpass filter circuit (hereinafter, BPF) 104 for extracting a carrier frequency component from the voltage signal having been amplified; a carrier detection circuit 105 for detecting a carrier from the carrier frequency component having been extracted; an integrating circuit 106 for integrating carrier-existing periods; and a hysteresis comparator 107 which compares an output of the integrating circuit 106 with a threshold level, thereby (i) judging whether or not the carrier exits and (ii) outputting the result of the judgment in the form of digital output. The digital output Dout of the hysteresis comparator 107 is sent to a microcomputer or the like which controls an electronic device.
FIG. 24 shows an output of each circuit of the infrared remote control receiver 110. FIG. 24(a) shows an output of the current signal Iin. FIG. 24(b) shows an output of the BPF 104 (solid line) and that of the carrier detection circuit 105 (dotted line). FIG. 24(c) shows an output of the integrating circuit 106 (solid line). FIG. 24(d) shows a digital output Dout of the infrared remote control receiver 110.
Note that the dotted line in FIG. 24(c) is a threshold level.
The infrared remote control receiver performs a high-gain amplification. As such, an influence from power source noise is remarkable. For this reason, power-source noise canceling characteristic PSRR (power supply rejection ratio) needs to be improved.
FIG. 25 shows a configuration of an infrared remote control receiver 120 whose power-source noise canceling characteristic is improved compared to the infrared remote control receiver 110. The infrared remote control receiver 120 has the same configuration as that of the infrared remote control receiver 110 except in that an operational amplifier circuit 103a is substituted for the amplifying circuit 103.
As shown in FIG. 26, the operational amplifier circuit 103a includes: a transconductance amplifier circuit (hereinafter simply referred to as GM) 111; a constant voltage circuit (VS)112, and an output load ZL.
The gain Av of the operational amplifier circuit 103a is:Av=(vout+−vout−)/(vin+−vin−)=gm*ZL 
where:
vin+ and vin− are differential input voltages of the operational amplifier circuit 103a; 
vout+ and vout− are differential output voltages of the operational amplifier circuit 103a; 
gm is a transconductance of the GM111; and
ZL is a load impedance.
As shown in the figure, in the operational amplifier circuit 103a, a power source voltage for the GM111 is an output voltage vref of the constant voltage circuit 112, and not a power source voltage vdd which could be varied due to power source noise. Therefore, the power-source noise canceling characteristic can be improved. However, when the output voltage vref of the constant voltage circuit 112 is used as the power source voltage for the GM111, the dynamic range is reduced by a difference between vdd and vref (i.e., vdd−vref). For example, since vdd−vref usually needs to be approximately 0.2V, the output voltage vref of the constant voltage circuit 112 is 2.2V when the power source voltage vdd is 2.4V. As such, the GM111 operates at 2.2V.
A battery-driven device is required to operate at a low voltage. However, since the operational amplifier circuit 103a adopting the constant voltage circuit 112 fails to ensure a sufficient dynamic range when operated at a low voltage, the operational amplifier circuit 103a is not suitable for a low-voltage operation. Further, it is crucial to ensure a sufficient dynamic range in an infrared remote control receiver. This is because a BPF whose output amplitude is large is arranged in a stage after the amplifying circuit.
For the above reasons, improving of a power-source noise canceling characteristic by means of the operational amplifier circuit 103a is not feasible in an infrared remote control receiver.